It's possible that it is a sensory issue. However, it could also be a circulation or other health issue. If you have circulation problems from sitting for prolonged periods this might happen. You may not even have realized that you have such a problem until you look into the floating issue. Either cause can be relieved by getting up and walking around for a few minutes whenever it happens. If that doesn't fix it, see a vascular doctor. Also have your blood pressure and blood sugar checked. And if there is nothing wrong, you may want to see a neurologist, as there may be a neurological problem, such as a tumor, or mini stroke, etc.
So get yourself checked out for safety's sake. 
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If a man does not keep pace with his companions, perhaps it is because he hears a different drummer.
Let him step to the music which he hears, however measured, or far away.--Henry David Thoreau